Electrodeless apparatus for producing or accelerating plasmoids



0, 1966 A KOLLER ETAL 3,270,236

ELECTRODELESS APPARATUS FOR PRODUCING 0R ACCELERATING PLASMOIDS Filed March 6, 1964 5 Sheets-Sheet 1 FIG. 2

0. 1966 A. KQLLER ETAL 3,270,236

ELECTRODELESS APPARATUS FOR PRODUCING 0R AUCELERATING PLASMOIDS Filed March 6, 1964 5 Sheets-Sheet 2 0. 1966 A. KOLLER ETAL 3,270,236

ELECTRODELESS APPARATUS FOR PRODUCING OR ACCELERATING PLASMOIDS Filed March 6, 1964 5 Sheets-Sheet 5 FIG. 4

United States Patent 3 270 236 ELECTRODELESS APPARATUS FOR PRODUCING OR ACCELERATING PLASMOIDS Alois Koller, Alfred Michel, and Heinrich Schindler,

Erlangen, Germany, assignors to Siemens-Schuclrertwerlre, Aktiengesellschaft, Berlin-Siemensstadt and Erlangen, Germany, a corporation of Germany Filed Mar. 6, 1964, Ser. No. 349,996 Claims priority, application Germany, Mar. 8, 1963, S 84,063 8 Claims. (Cl. 313153) Our invention relates to apparatus for producing or accelerating plasmoids without the aid of electrodes by means of electromagnetic fields.

While an electrically neutral gas can be accelerated only by a pressure gradient, a second possibility, namely the Lorentz force, is available for the acceleration of plasmas. The effect of this force can be understood as follows.

With reference to a cartesian system of coordinates,

assume that the vector of a magnetic field strength H be in the z-dire-ction and the vector of an electric field strength E in the y-d'irection, bot-h fields being homogeneous. It follows from the analysis of the motion performed by an electrically charged particle, not colliding with other particles, that its velocity component in the direction of the magnetic field lines remains unchanged. In the x-y plane the particle travels on a cycloid. That is, it performs a circular motion while the center of the circle travels at the speed in the x-di-rection and hence perpendicular to both the electrical and magnetical field lines. In the equation, E denotes the value of the electrical field strength and B the value of the magnetic induction. The drift velocity is independent of the sign (polarity) of the electric charge and also independent of the particle mass. The velocity component in the y-direction, on the average, is equal to zero.

When a plasma is located in a field with mutually intersecting electrical and magnetic field lines, each individual particle thus travels at the speed v in the x-di rection, provided no collisions occur between the particles. No current can then flow in the y-direction. When collisions occur, the cycloid motion of the individual particles is repeatedly interrupted. However, by statistical averaging the motions of all individual particles permits expressing the motion by the simple equation The term grad p indicates that a plasma can also be accelerated by a pressure gradient. The term i x B represents the Lorentz force and has the meaning that a plasma is always accelerated in a direction perpendicular to i and B whenever a current component i perpendicular to B is present. The term i is detenmined by Ohms law. If no collisions occur, the acceleration terminates at the moment when the plasma attains the speed because then a net motion in the direction of the electrical field strength and hence an electrical current no longer exists.

The available literature ofiers reports on several devices which utilize the Lorentz force for the acceleration of plasmas. In this connection, reference may be had to the publication by Marshall in Proceedings 2nd U.N. Intern. Conf. on the Peaceful Uses of Atomic Energy, Geneva, 1958, 32, 341 (1959); Shpigel in Sov. Phys.

JETP. 36, 285 (1959); Htigberg und Vogel in Nucl. Inst. 10, (1961); Waelbroeck et al. in Nucl. Fusion Suppl., Part 2, 675.

Our invention resides in a novel device for producing and/ or generating plasma by utilization of the Lorentz force, which affords considerable advantages over those heretofore known.

According to our invention, we coaxially mount two magnetic field coils of mutually opposed magnetic action, for example mutually opposed winding sense, upon a tubular vessel of insulating material in axially spaced relation to each other so that, when the two coils are electrioally excited, a magnetic field having a radial component (cusp field) is produced in the space between these two field coils. We further provide between the two field coils a coaxial low-induction winding and excite this Winding by periodic or pulsating current. As a result, the correspondingly varying magnetic field of the winding induces in the interior of the tubular vessel a circular electric field which, when the tubular vessel contains gas, produces plasma and generates therein a ring flow of electric current in the radial magnet field. The circulating flow of current, together with the magnetic cusped field of the two field coils, exerts an accelerating force upon the plasma, and this force has a component in the axial direction of the tubular vessel. The axial component of the magnetic field of the induction wind-. ing further effects a compression of the plasma toward the axis.

Compared with the electrodeless plasma guns heretofore known, the device according to the invention alfords the advantage that the plasma can be selectively ejected or injected in either of the two axial directions, a result heretofore not achieved with any of the known and otherwise comparable plasma devices. The field lines of the cusp field, at some distance from the intermediate winding, extend parallel to each other, and thus form simultaneously the guiding field.

According to another feature of our invention, two or more coil and winding groups, as described above, are arranged in series upon a single tubular vessel. This aifords subjecting an accelerated plasma to further acceleration or to retardation as well as to reflection. This permits an operation accodring to which a plasmoid is periodically thrown back and forth between two coil groups with the effect of thus dynamically confining the plasma with the aid of variable magnetic fields.

According to still another feature of our invention, the pulse current which energizes the low-induction winding is subjected to critical or supercritical damping. This can be done, for example, by connecting a damping resistor serially into the excitation circuit of the induction winding. Such critical or supercritical damping in conjunction with an intensive pre-ionization of the gas, for example by high frequency, oifers the possibility of accelerating an individual plasmoid at a given moment in a predetermined direction. The pro-ionization is necessary to have the current in the gas space commence to flow at sufficient intensity simultaneously with the flow of pulse current in the induction coil. The series-com nected resistor may be substituted by a resistor or other impedance which is either continuously connected in parallel to the induction winding or which is switched into action at the moment of the current-zero passage, the switching being effected by a suitable trigger or releasing device, for example a spark gap.

The invention will be further explained with reference to embodiments illustrated by way of example on the accompanying drawings and described presently. In the drawing:

FIG. 1 shows schematically and in diametrical section a plasma-producing and accelerating device;

FIG. 2 shows schematically a similar device in conjunction with a circuit diagram of the appertaining electric energizing circuits, and FIG. 2a shows a modified portion of FIG. 2.

FIG. 3 shows schematically and in cross section a multiple device according to the invention; and

FIG. 4 shows a modified device in section and partially in perspective.

Denoted by 11 and 12 in FIG. 1 are two annular field coils which are coaxially aligned and surround a tubular vessel 14 of circular cross section. The field coils 11 and 12 are axially spaced from each other and have a mutually opposed winding sense So that when they are simultaneously excited by electric current, a magnetic field having a radial component, a so-called cusp field, is produced inside the tubular vessel 14 in the space between the two field coils. The field lines of the cusp field are exemplified by those schematically shown at 15.

Mounted :between the two field coils and likewise coaxially related to the tubular vessel 14 is a low-induction winding 13. This winding has only a few turns or only a single turn of large cross section so as to have a correspondingly high current-carrying capacity. During operation, the inductance winding 13 is energized by powerful current surges which have a steeply increasing front and thus rapidly increase each time from zero to full intensity. The correspondingly varying magnetic field of the inductnace winding 13 induces a circular electrical field in the interior of the insulating tube 14, this electrical field being schematically indicated at 16. When a neutral gas, for example hydrogen, is contained in the tube 14, the gas becomes ionized in one of the first few half waves. Now an electric current commences to circulate on a ring-shaped path within the radial magnetic field.

Referring to cylinder coordinates, this circulating ringpath current results in a current i in the azimuth direction and in a magnetic induction B having a component in the radial direction. The axial component of the resulting Lorentz force drives the plasma in the direction of the tube axis, this being schematically indicated by an arrow 17, the direction of motion being away from the acceleration center. The electrons and ions then travel in the homogeneous magnetic field of the field coil 12. This field simultaneously acts as a guiding field because it opposes the diffusion of the charge carriers toward the wall of the tubular vessel. Since the magnetic field of the induction winding 13 has an axial direction, there also exists a radial acceleration component toward the axis.

When the direction of either the electrical or the magnetic field is rotated 180, the axial component of the acceleration vector also rotates 180. Consequently by combining a plurality of acceleration centers, a repeated acceleration or reflection of a plasmoid can be obtained. This is an essential advantage afforded by the invention over the plasma accelerators heretofore known.

The locality at which the acceleration takes place in a device according to the invention is freely accessible from both sides of the tube for entering or issuing plasma. In all other devices with a predetermined ejection direction heretofore describe-d, one side of the tube is either fully closed or must possess only a smaller opening than the side toward which the plasma is being accelerated.

In the circuit diagram of FIG. 2, the field coils for producing the guiding and cusped field are denoted by 21. They are electrically connected in series. Denoted by 23 is the low-induction winding for producing the circular electric field. The coaxial coils and windings surround an insulating tube 24 of glass filled with hydrogen under 0.8 mm. Hg pressure. The electric equipment comprises an ignition device 25, three spark gaps 26, 27 and 28, a time delay stage 29, a Rogowski belt (current transformer) 30', a av-probe 210, resistors 2:11, 212 and 217 which are preferably adjustable, and capacitors de- 4 noted by 213, 214, 215 and 216. A damping resistor 31 is connected in the circuit of the low-inductance (theta) winding 23 to critically or supercritically damp the current surges passing therethrough.

The axial length of the respective field coils 21 and 22 is made different in order to obtain on one side a long travel path of the accelerated charge carriers. The winding 23 consists of a single turn to keep the self-induction low and thereby the rate of current increase correspondingly high.

The field strength of the guiding field should attain highest possible values of reducing the diffusion of charge carriers to the glass wall during travel through the tube. For that reason a current of a few hundred amperes, for example 300 to 400 amps, is passed through the coils 21 and 22. This current must be supplied in periodic pulses or surges to prevent overloading of the coils. The pulsating operation affords producing a strong magnetic field without appreciable heating of the coils. In the homogeneous portion of the guiding field, field strength values of 5000 gauss have been measured. The time con stant of the field coil energizing circuit is chosen to be large in comparison with the duration of the discharge so that the guiding field can be looked upon as being quasistatic.

The operation of the apparatus according to FIG. 2 is as follows.

First the spark gap 26 in the energizing circuit of the field coils is triggered by the ignition device 25. Simultaneously, a capacitor 216 causes operation of the spark gap 27 which closes the circuit for the auxiliary field of the av-probe 210 for measuring the product of speed times conductance of the plasma. The increase of this current causes a voltage to be induced in the Rogowski belt 30 which initiates the functioning of the time delay stage 29. The delay line 29 ignites the spark gap 28 in the circuit of the low-induction winding 23 when the magnetic fields of the coils 21 and 22 have reached their maximal value. During the subsequent acceleration operation, the magnetic field does not measurably change its value because of the long time constant in comparison with the short and steep high-current pulse applied to the winding 23. The proper relation of the time constants required for the timing just described is secured by suitable choice or adjustment of the resistors 211, 212 and 217.

The somewhat modified circuit portion shown in FIG. 2 comprises a resistor 32 in parallel connection to winding 23, and a switching or trigger device 33, such as a spark gap, for closing the parallel circuit of resistor 32. Resistor 32 is low ohmic and may be made zero if switching is to occur exactly at the current zero passage.

With the equipment described above with reference to FIG. 2, optical and electrical speed measurements of the accelerated plasma have been performed. For optical measurements, openings were left vacant in the field coils through which the light issuing from the plasma could emerge. Sweep pictures of the slot-shaped openings were taken with the aid of an image converter. The openings were shaped as slits and were arranged sequentially in the accelerating direction. Since the slot spacing was known, the time spacing a which they became sequentially lit, permitted determining the speed of the plasma. The speed was found to be 2 to 310 cm./sec.

The av-probe was mounted at different distances from the acceleration center. The electric speed measurement with the aid of the signal increase of the av-probe likewise resulted in a plasma speed of 2 to 3 1O cm./sec. Together with these speed measurements, the electric conductance of the plasma could be determined from the amplitude of the measuring signal issuing from the avprobe. The conductance was thus found to be 50 ohm- CITLTI.

The above-described measuring operations were performed with parameter values ginven presently by way of example, it being understood that these values can be varied or modified depending upon the particular circuitry used as well as upon the particular test purposes pursued.

Coils 21 and 22 had 80 and 240 turns respectively, the respective circuits had a natural frequency of about 100 kHz. The energizing current pulses decayed with a time constant of 2-3 millisec.

The single-turn (theta) winding 23 was operated with surges of maximal 100 ka. and a time constant of decay within about microseconds.

The operation requires capacitor 216 to be in the nanofarad range, with capacitors 213-215 between 4 and microfarad. Resistor 31 must be low-ohmic (0.1-1 ohm), resistor 212 about ohms, with resistors 211 and 217 in the kilo-ohm range.

For checking, the test conditions were modified. A reversal of the guiding field must result in accelerating the plasma in the opposite direction. Similarly, a directional change in acceleration must also take place in each half-wave period of the current surge in the low-induction winding 23. Both phenomena were indeed measured optically .as well as electrically.

The plasma vessel and the coils may also be given a torus-shaped arrangement, for example in such a manner that the field lines assume the shape of a distorted or bumpy torus for the purpose of preventing the toroidal drift phenomena. Such a torus arrangement affords the possibility of obtaining simple and multiple plasma acceleration, plasma reflection, the theta pinch, as well as static and dynamic reflection fields and cusp fields, and also combining such phenomena.

The embodiment shown in FIG. 3 by a diametrical cross section exemplifies a toroidal arrangement of four plasma guns. The four field coils I, II, III and IV shown by heavy lines constitute the theta coils, the other coils produce the cusped and guiding fields. In this arrangement the plasma can be kept in a circular motion and can be continually accelerated. The resulting advantage is the fact that the plasma diflusing through the guiding field is each time newly compressed by the theta coil so that it does not reach the vessel Wall. The five coils of the guiding field between each two theta coils can be designed as a single coil. However, the provision of separate guiding coils renders the device more versatile because this affords a free choice with respect to selectively using a theta-operation coil after each two or three guiding coils instead of after five guiding coils.

Such a coil arrangement of toroidal shape is highly versatile, as will be apparent from the following description of a few possibilities of application.

The plasma can be shot back and forth between the coils I and II. During this interval of time, the coils III and IV, as well as the intermediate guiding coils and the appertaining electric circuits, can recuperate from the preceding operation (for example by cooling, de-ionizing of the spark gaps). As soon as the confinement of the plasma between coils I and II becomes doubtful or critical by occurrence of losses, the plasma is shot into the space between coils II and III. Now .the space between coils I and IV remains inactive. This amounts to a method of dynamically confining the plasma. The resting period for the coils can be prolonged by providing more than 4 theta coils.

Instead of the cusped fields, reflection fields can be produced at suitable localities, for example at the coil III and the two adjacent coils; and the plasma can then be shot by the gun at 11 into the reflecting field. It is further possible to thereafter produce a theta pinch by suddenly increasing the current in coil III. The pinch heats the plasma to a higher temperature and drives itacting as a theta-pinch guntoward coils II and IV where the plasma can be further accelerated until it again arrives at coil I.

If each coil is energized by a current of the same flow direction as the one passing through the adjacent coils, but if, for example, each third coil is supplied with a stronger current, then a sequence of reflecting fields is produced around the torus. The totality of these reflecting fields assume a M and S configuration (bumpy torus, Meyer and Schmidt) which exhibits more favorable plasma confining properties than the purely toroidal magnetic field. More particularly, .a toroidal theta pinch can be produced along the entire torus in this manner.

Furthermore, toroidal z-pinches are also obtainable by suitable modification. For this purpose, an iron core 23 is placed between two coils 21, 22, as shown in FIG. 4, the windings 24 of an external circuit 25 being wound about the iron core. When a current pulse is simultaneously passed in the same sense through all of these windings, the plasma in the interior 26 of the torus acts as a transformer secondary Winding in which a strong current is induced resulting in a toroidal z-pinch. The iron rings can be omitted if the coils themselves are made of ferromagnetic material and are employed in lieu of the iron rings in the same manner as the latter. The z-current may also be passed through current-carrying windings parallel to the torus core (axis).

In this manner, several dilferent methods for confinement of plasma can be employed in one and the same device, and such confining methods may also be combined with each other or may be used alternately in a desired time sequence.

We claim:

1. Apparatus for electromagnetic control of plasmoids, comprising a tubular vessel of electrically insulating material which contains gas when in operation, two magnetic field coils magnetically opposed to each other and surrounding said tubular vessel in coaxial relation thereto and mutually spaced axially for producing between each other a magnetic field having a radial component, a lowinduction winding coaxially surrounding said tubular vessel between said two field coils, electric energizing means connected to said field coils, and periodic electric energizing means connected to said inductance winding for causing the periodic magnetic field of said inductance winding to produce in the vessel a circular electric field, whereby the electric field generates plasma and therein a ring-flow of current in the radial magnetic field resulting in axially directed acceleration of the plasma.

2. In plasma control apparatus according to claim 1, said induction-winding energizing means comprising a current surge supply circuit connected with said Winding, and a damping impedance in said circuit for at least critically damping the current in said winding.

3. Apparatus for electromagnetic control of plasmoids, comprising a tubular vessel of electrically insulating material which contains gas when in operation, two magnetic field coils magnetically opposed to each other and surrounding said tubular vessel in coaxial relation thereto and mutually spaced axially for producing between each other a magnetic field having a radial component, a lowinduction winding coaxially surrounding said tubular vessel between said two field coils, electric energizing means connected to said field coils, and periodic electric energizing means connected to said inductance winding for causing the periodic magnetic field of said inductance winding to produce in the vessel a circular electric field, whereby the electric field generates plasma and therein a ring-flow of current in the radial magnetic field resulting in axially directed acceleration of the plasma, said energizing means for said field coils comprising pulse generating means, and said induction-Winding energizing means comprising a current-surge supply circuit of short surge duration compared with that of said pulse-generating means and trigger means in said surge circuit for releasing surges in a given time relation to said pulses.

4. In plasma control apparatus according to claim 3, said trigger means comprising adjustable resistance means for varying the time point of surge release relative to said pulses.

5. Apparatus for electromagnetic control of plasmoids,

comprising a tubular vessel of electrically insulating material which contains gas when in operation, a plurality of coil groups of which each comprises two magnetic field coils magnetically opposed to each other and surrounding said tubular vessel in coaxial relation thereto and mutually spaced axially for producing between each other a magnetic field having a radial component, a low-induction winding coaxially surrounding said tubular vessel between said two field coils, said coil groups being disposed in axially sequential relation, electric energizing means connected to said field coils, and periodic electric energizing means connected to said inductance winding for causing the periodic magnetic field of said inductance winding to produce in the vessel a circular electric field, whereby the electric field generates plasma and therein a ring-flow of current in the radial magnetic field resulting in axially directed acceleration of the plasma.

6. Apparatus for electromagnetic control of plasmoids, comprising a tubular vessel of electrically insulating material which contains gas when in operation, said tubular vessel having toroidal shape, a plurality of coil groups of which each comprises two magnetic field coils magnetically opposed to each other and surrounding said tubular vessel in coaxial relation thereto and mutually spaced axially for producing between each other a magnetic field having a radial component, a low-induction winding coaxially surrounding said tubular vessel between said two field coils, said coil groups being disposed in axially sequential relation, electric energizing means connected to said field coils, and periodic electric energizing means connected to said inductance winding for causing the periodic magnetic field of said inductance winding to produce in the vessel a circular electric field, whereby the electric field generates plasma and therein a ring-flow of current in the radial magnetic field resulting in axially directed acceleration of the plasma.

7. Apparatus for electromagnetic control of plasmoids, comprising a tubular vessel of electrically insulating material which contains gas when in operation, two magnetic field coils magnetically opposed to each other and surrounding said tubular vessel in coaxial relation thereto and mutually spaced axially for producing between each other a magnetic field having a radial component, an annular core of ferromagnetic material coaxially disposed between said two field coils, a low-induction winding coaxially surrounding said tubular vessel between said two field coils, electric energizing means connected to said field coils, and periodic electric energizing means connected to said inductance winding for causing the periodic magnetic field of said inductance winding to produce in the vessel a circular electric field, whereby the electric field generates plasma and therein a ring-flow of current in the radial magnetic field resulting in axially directed acceleration of the plasma.

'8. Apparatus for electromagnetic control of plasmoids, comprising a tubular vessel of electrically insulating material which contains gas when in operation, two magnetic field coils magnetically opposed to each other and surrounding said tubular vessel in coaxial relation thereto and mutually spaced axially for producing between each other a magnetic field having a radial component, a low-induction winding coaxially surrounding said tubular vessel between said two field coils, electric energizing means connected to said field coils, and periodic electric energizing means connected to said inductance winding for causing the periodic magnetic field of said inductance winding to produce in the vessel a circular electric field, resistance means and a controllable switching device connected in series with each other across said low-inductance winding for short-circuiting said winding through said resistance means at a selected moment under control by said switching device, whereby the electric field generates plasma and therein a ring-flow of current in the radial magnetic field resulting in axially directed acceleration of the plasma.

References Cited by the Examiner UNITED STATES PATENTS 11/1964 Hill 315-11l 1/1965 Leboutet 3l5-111 

1. APPARATUS FOR ELECTROMAGNETIC CONTROL OF PLASMOIDS, COMPRISING A TUBULAR VESSEL OF ELECTRICALLY INSULATING MATERIAL WHICH CONTAINS GAS WHEN IN OPERATION, TWO MAGNETIC FIELD COILS MAGNETICALLY OPPOSED TO EACH OTHER AND SURROUNDING SAID TUBULAR VESSEL IN COAXIAL RELATION THERETO AND MUTUALLY SPACED AXIALLY FOR PRODUCING BETWEEN EACH OTHER A MAGNETIC FIELD HAVING A RADIAL COMPONENT, A LOWINDUCTION WINDING COAXIALLY SURROUNDING SAID TUBULAR VESSEL BETWEEN SAID TWO FIELD COILS, ELECTRIC ENERGIZING MEANS CONNECTED TO SAID FIELD COILS, AND PERIODIC ELECTRIC ENERGIZING MEANS CONNECTED TO SAID INDUCTANCE WINDING FOR CAUSING THE PERIODIC MAGNETIC FIELD OF SAID INDUCTANCE 